fix: normalize finish reason in CreateResult response #3572

Job	Run time
docs-example-check (./packages/autogen-core)	2m 41s
docs (./packages/autogen-core)	1m 31s
check-proto-changes-python	1m 10s
codecov (./packages/autogen-core)	1m 7s
codecov (./packages/autogen-magentic-one)	1m 51s
format	48s
lint	52s
codecov (./packages/autogen-ext)	3m 22s
markdown-code-lint	2m 0s
codecov (./packages/autogen-agentchat)	1m 18s
mypy (./packages/autogen-core)	2m 35s
pyright (./packages/autogen-core)	57s
mypy (./packages/autogen-magentic-one)	1m 27s
samples-code-check	1m 1s
test (./packages/autogen-core)	57s
mypy (./packages/agbench)	1m 5s
pyright (./packages/autogen-magentic-one)	56s
test (./packages/autogen-magentic-one)	1m 25s
mypy (./packages/autogen-ext)	1m 55s
pyright (./packages/agbench)	52s
test (./packages/autogen-ext)	3m 4s
mypy (./packages/autogen-agentchat)	1m 32s
pyright (./packages/autogen-ext)	1m 3s
test (./packages/autogen-agentchat)	58s
mypy (./packages/magentic-one-cli)	1m 22s
pyright (./packages/autogen-agentchat)	1m 5s
pyright (./packages/magentic-one-cli)	1m 4s
	39m 58s

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